The basic principles involved in electrical exploration, and in particular the use of the earth resistivity method, are rather well understood and have been known for many years. Briefly, the fundamental concept can be understood by visualizing a source of voltage or current connected to two electrodes which are connected to the earth at two spaced-apart locations. When energized, current will flow between the electrodes through the earth and that current will follow many paths (theoretically an infinite number). In an idealized situation where the subsurface structure is homogeneous, in the local vicinity of either current electrode the current will establish equipotentials on generally hemispherical concentric surfaces or shells the edges of which will intersect the earth's surface. The magnitudes of the potential measured are functions of the resistivity of the subsurface material and their distances from the source electrode. Thus, measurement of the potentials at the earth's surface permits one to infer data about the earth's substructure materials.
In a real situation, the substructure is not homogeneous throughout any region of significant size. However, it is possible to introduce corrections to account for changes in resistivity which result from strata which cause distortion of the otherwise hemispherical equipotential surfaces or shells. Of more interest, it is possible to identify resistivity anomalies which are very different from the surrounding rock because they consist of concentrations of ores, gas, water or petroleum. Particularly noteworthy is the fact that the resistivity of a petroleum deposit is often different from the surrounding rock. Examples and discussions of prior art techniques and systems can be found in the following documents.
U.S. Pat. No. 1,841,376, Nichols et al
U.S. Pat. No. 2,138,818, Jakosky
U.S. Pat. No. 2,172,778, Taylor, Jr.
U.S. Pat. No. 2,231,048, Beacham
U.S. Pat. No. 2,390,270, Piety
U.S. Pat. No. 2,531,088, Thompson
U.S. Pat. No. 2,920,266, Owen
U.S. Pat. No. 3,256,480, Runge et al
U.S. Pat. No. 3,329,929, Burnett
U.S. Pat. No. 3,344,342, Kinghorn
U.S. Pat. No. 3,525,037, Madden et al
U.S. Pat. No. 3,621,380, Barlow, Jr.
U.S. Pat. No. 3,659,192, Ryss et al
U.S. Pat. No. 3,737,768, Lazenby et al
U.S. Pat. No. 3,810,000, Aspinall et al
U.S. Pat. No. 3,984,759, Gaucher et al
U.S. Pat. No. 3,975,674, McEuen
U.S. Pat. No. 3,975,676, Bliamptis
U.S. Pat. No. 4,041,372, Miller et al
U.S. Pat. No. 4,258,323 Andrews et al
French No. 1,370,777, Fernandez-Bollo
Subsurface Void Detection Using Surface Resistivity And Borehole Electromagnetic Techniques-Owen et al, Society of Exploration Geophysicists, 50th Annual Meeting of SEG, Houston, Tex., October 1981 PA1 Use Of An Automatic Earth Resistivity System For Detection Of Abandoned Mine Workings-Peters et al, Society of Mining Engineers of AIME (Paper, AIME meeting, Feb. 22-26, 1981) PA1 Modeling Resistivity Anomalies From Localized Voids Under Irregular Terrain-Spiegel et al, GEOPHYSICS, Vol. 45, No. 7, July, 1980
As will be seen, these documents discuss a wide variety of techniques including surface and borehole measurements. They illustrate the many approaches which have been tried and used, and many of them have been successful, especially as an adjunct to other exploration techniques.
For several reasons, however, surface (i.e., not borehole) resistivity exploration has not been as useful as other methods, particularly for petroleum exploration purposes. One reason is that the method is inherently depth-limited, normally being dependent upon the size and electrical contrast of the target deposit.
Another reason relates simply to the practical problems of handling the equipment in the field. As will be seen from the above references, one must handle long and relatively cumbersome electrical cables which interconnect the current injecting electrodes and the current source, and one must also provide measuring equipment. Furthermore, this equipment must be, in part, highly mobile to make the measurements, and the probes and cables must be frequently relocated to execute the survey. Because the equipment of the prior art is difficult to handle, much time has been required to make measurements, causing the procedure to be expensive.
Perhaps the most significant factor, however, is the lack of sufficient accuracy. Partly because the equipment is hard to handle, only relatively few measurements are normally made which means that insufficient data is gathered to permit accurate prediction of anomaly locations or characteristics. And finally, the amount of data processing required to analyze the data from even a few detector/emitter combinations is very laborious and time consuming.